1. Field of the Invention
The present invention relates to an image processing apparatus for detecting and removing noise that is included in an image resulting from decoding image data obtained through lossy coding, such as the MPEG (Moving Picture Experts Group) system, and further relates to a method thereof.
2. Description of the Related Art
Conventional image coding schemes include Motion JPEG, which is an intraframe coding scheme; coding schemes for digital video or the like; and H.261, H.263, MPEG-1, and MPEG-2, which use inter-frame predictive coding. In addition, recently, coding schemes such as H.264 are also known. For these coding schemes, international standards have been established by the ISO (International Organization for Standardization) and the ITU (International Telecommunication Union).
MPEG-2, which is a representative coding scheme, is called “lossy coding”, and the image data coded with this coding scheme cannot be perfectly restored by decoding. This is because at the time of coding, quantization is performed after a DCT transformation. These coding schemes reduce the amount of data by removing redundancy included in the subject signals by utilizing the statistical nature of image signals. In other words, since human vision is not sensitive to high-frequency components of images, redundancy is removed by roughly quantizing such high-frequency components so as to achieve a high coding efficiency.
However, in a lossy coding in which this type of DCT transformation is performed, data is compressed by limiting high-frequency components, and thus noise called “mosquito noise” occurs on the edge portions of images or around moving objects, and this noise causes reduced image quality in the decoded image. Methods for reducing such mosquito noise have been proposed, and in Japanese Patent Laid-Open No. 2001-218210, a method for obtaining an image signal with reduced mosquito noise by detecting an edge portion of an input image signal and filtering the edge portion of the image signal with a noise removal filter has been proposed. For the details of other items, such as the MPEG system, see the international standards established by ISO/IEC.
However, in the above-mentioned conventional art, since the filtering process for removing mosquito noise is performed on edge portions of image signals regardless of the presence or absence of mosquito noise in the image signals, portions without mosquito noise suffer from image deterioration. In addition, when a texture area that includes fine edges is included, the area is erroneously determined to be an edge, and the texture information is removed. The result is a reduction of the image quality.
These problems are due to noise removal process being performed on edge portions of image signals without accurately detecting the noise portions, such as mosquito noise, included in the image obtained through decoding of frequency-transformed or lossy coded image data. The problems are also due to noise removal processes being performed by considering texture areas of image signals that include fine edges to be noise portions.